Potentially looking to get an Asus RT-N16 to replace an ageing (yet still very reliable) WRT54GL and step up from Tomato v1.19.

Do any of the builds currently available have SNMP support built in (I read somewhere that the VPN build might?)? Purely used for tracking bandwidth stats etc. Would rather not have to mess around with Optware etc.

You'll definitely want to use a tomatousb mod. So far they're the only ones I know of with snmp. I believe Shibby, creator of the Shibby mod, made this possible. Other mods like Toastman's mod have also incorporated snmp as well. You may find other mods that have it also in the 'Tomato USB Modifications' sub-forum. Both these mods are very actively developed, there hasn't been an official tomatousb release for a year, though it's creator TeddyB has contributed since then to it's development.